Automatic dosing cap, which is provided for the automatic dosing of a fluid (2) any contained in a flexible container (1), is characterized in that said container (1) is provided with a fixed plug (3) in its lower area or lateral whose inner shape is hollow and cylindrical of parallel walls, having a central hole (4) for communication to the outside and of smaller diameter than the rest of the main cylindrical cavity; inside which is installed a movable cylinder (5) whose outer shape is integral with the diameter of the interior of the main cavity of the cap (3), ending in a neck (6) with holes (10) at its lateral end, whose shape is in turn integral with that of the central hole (4) of the plug (3); in such a way that a certain tolerance between the body forming a movable cylinder (5) and the neck (6) thereof, and the walls of the cap (3) allow the free movement of the first without a fluid (2) slipping between your walls the interior of said movable cylinder (5) is determined as a new cylindrical cavity with parallel walls provided in its upper middle area with several communicating windows (7) of said cavity with the interior of the container (1), in order to allow the passage of a portion of fluid (2) contained in the container (1) therein; in turn, the movable cylinder (5) is internally provided with a piston (8) disposed above said communicating windows (7) whose outer diameter is integral with that of the interior of the moving cylinder (5) in such a way that a certain tolerance between said piston (8) and the walls of the movable cylinder (5) allow the displacement thereof without a fluid (2) sliding between both walls. The piston (8) is held in this position by the action of a coil spring (9) installed inside the cavity of the movable cylinder (5) and by a stop (17) of which the upper neck of the movable cylinder (5) is provided. ) that prevents its exit from the device by the thrust exerted by said spring (9). Said piston (8) determines the outlet to the outside through the holes (10) of the neck (6) of the movable cylinder (5), of a portion of fluid (2) contained inside the cavity of said movable cylinder ( 5), after a force (p-1) exerted on the container (1) is transmitted dynamically by the fluid (2) to this mobile cylinder (5) through the communicating windows (7) and has displaced it and blocked, when determining the transmission of the resulting force (p-2) to the piston (8), which will push the fluid (2) to the outside through the holes (10) once the communicating windows (7) have remained covered by the inner wall of the plug (3). The recovery of the original shape of the container (1) will determine a new vacuum force (v-1) contrary to the first determining the opposite movement of the piston (8) towards the upper part of the cylindrical cavity of the moving cylinder (5), causing the air intake inside it through the holes (10), which will fill said cavity; a final vacuum force (v-2) will determine the retraction of the movable cylinder (5) to its initial position, then determining the passage of air into the container (1) through the communicating windows (7) of the moving cylinder ( 5), in order to be exchanged for another dose of fluid (2) that will again fill the cavity of the movable cylinder (5). (Machine-translation by Google Translate, not legally binding)